Investigating spatial macroscopic metastability of perovskite solar cells with voltage dependent photoluminescence imaging

Metastability is a characteristic feature of perovskite solar cell (PSC) devices that affects powerrating measurements and general electrical behaviour. In this work the metastability of differenttypes of PSC devices is investigated through current–voltage (I–V) testing and voltage dependentphotoluminescence (PL-V) imaging. We show that advanced I–V parameter acquisition methodsneed to be applied for accurate PSC performance evaluation, and that misleading results can beobtained when using simple fast I–V curves, which can lead to incorrect estimation of cellefficiency. The method, as applied in this work, can also distinguish between metastability anddegradation, which is a crucial step towards reporting stabilised efficiencies of PSC devices. PL-V isthen used to investigate temporal and spatial PL response at different voltage steps. In addition tothe impact on current response, metastability effects are clearly observed in the spatial PL responseof different types of PSCs. The results imply that a high density of local defects andnon-uniformities leads to increased lateral metastability visible in PL-V measurements, which isdirectly linked to electrical metastability. This work indicates that existing quantitative PL imagingmethods and point-based PL measurements of PSC devices may need to be revisited, asassumptions such as the absence of lateral currents or uniform voltage bias across a cell area maynot be valid

Enhancing fully printable mesoscopic perovskite solar cell performance using integrated metallic grids to improve carbon electrode conductivity

Carbon based Perovskite Solar cells (C–PSCs) have emerged as the most promising candidates for commercialisation in the field of perovskite photovoltaics, as they are highly stable, low cost and make use of easily scaled manufacturing techniques. However, the limited conductivity of the carbon electrode inhibits performance and represents a significant barrier to commercial application. Τhis work presents a scalable method for enhancing the carbon electrode conductivity through the integration of aluminium and copper grids into prefabricated C–PSCs. Adhered to the cells using an additional low temperature carbon ink, the metallic grids were found to dramatically reduce top electrode series resistance, leading to a large improvement in fill factor and efficiency. After grid integration, the 1 cm2 C–PSCs yielded power conversion efficiency (PCE) of 13.4% and 13% for copper and aluminium respectively, while standard C–PSCs obtained PCE of 11.3%. Performance is also significantly augmented in the case of larger-scale 11.7 cm2 modules, where PCEs went from 7.7% to 10% and 11% for aluminium and copper grids respectively. This technique offers a fast and low temperature route to high-performance, large-area C–PSCs and could therefore have serious potential for application to the high-volume manufacture of perovskite cells and modules

Ultrathin uniform platinum nanowires via a facile route using an inverse hexagonal surfactant phase template

We present an attractive method for the fabrication of long, straight, highly crystalline, ultrathin platinum nanowires. The fabrication is simply achieved using an inverse hexagonal (HII) lyotropic liquid crystal phase of the commercial surfactant phytantriol as a template. A platinum precursor dissolved within the cylindrical aqueous channels of the liquid crystal phase is chemically reduced by galvanic displacement using stainless steel. We demonstrate the production of nanowires using the HII phase in the phytantriol/water system which we obtain either by heating to 55 °C or at room temperature by the addition of a hydrophobic liquid, 9-cis-tricosene, to relieve packing frustration. The two sets of conditions produced high aspect nanowires with diameters of 2.5 and 1.7 nm, respectively, at least hundreds of nanometers in length, matching the size of the aqueous channels in which they grow. This versatile approach can be extended to produce highly uniform nanowires from a range of metals

γ‐Valerolactone: A Nontoxic Green Solvent for Highly Stable Printed Mesoporous Perovskite Solar Cells

Mesoscopic carbon-based lead halide perovskite solar cells (CPSCs) represent a promising architecture for commercialization in the field of perovskite photovoltaics as they are stable, potentially low cost, and use easily scaled production methods. However, the use of toxic and psychoactive solvents such as dimethylformamide (DMF) and γ-butyrolactone (GBL) currently limits their commercial viability: DMF introduces a significant health risk and GBL is subject to legal restrictions in many countries. The development of safe and effective solvent systems is therefore an essential step toward commercial viability. Herein, γ-valerolactone (GVL) is presented as a nontoxic, biodegradable, green alternative to GBL for CPSC fabrication. Cells fabricated with a precursor concentration of 1.1 m and annealed at 45 °C exhibit comparable performance to standard GBL devices, achieving a champion power conversion efficiency (PCE) of 12.91% in a device of 1 cm2 active area. Herein, it is proven that GVL is a viable alternative to GBL for CPSCs and enables research in countries where GBL is legally restricted and makes large-scale CPSC manufacture more sustainable

Development and amelioration of green solvent systems for printed mesoscopic carbon perovskite solar cells

Since their advent in 2009, lead halide perovskite solar cells have rapidly progressed to exhibit power conversion efficiencies of 25.5%, approaching that of commercial monocrystalline silicon devices. Although cheap and amenable to solution processing, commercialization is currently limited by poor device stability under operating conditions, prohibitively expensive or toxic components and manufacturing methods unsuitable for large-scale production.Highly stable mesoscopic carbon-based perovskite solar cells (CPSCs) make use of easily scaled screen printing and are frequently described as one of the frontrunners for commercialization. However, significant barriers to commercialisation still exist. For example, the most common precursor solvents, dimethylformamide mixtures and γ-butyrolactone, respectively introduce toxicity and legality issues.This work presents the first application of γ-Valerolactone as a green, non-toxic alternative solvent for CPSC fabrication. Cells fabricated with optimised precursor concentrations and annealing conditions exhibit comparable performance to standard γ-butyrolactone devices, proving that this system is a viable alternative. This will enable continued research in countries where γ-butyrolactone is legally restricted and make large-scale CPSC manufacture more sustainable.This is then ameliorated with the application of green solvent engineering, wherein methanol (MeOH) is used as a solvent additive to improve the performance and reproducibility of GVL precursors. An optimised MeOH proportion of 10% is found to reduce precursor viscosity and improve wetting, as well as promoting more oriented crystal growth and higher quality absorber layers. Stability is also improved, with an unencapsulated MeOH device exhibiting a T80 of >420 hours at 50°C in ambient humidity under AM1.5 illumination.Post crystallisation humidity treatments and age-related performance enhancements are then examined. It is revealed that humidity treatments, required for hysteresis reduction in GBL cells, have no significant impact on GVL-MeOH devices. Age-related performance enhancements are instead found to be a consequence of crystal reorganisation due to slow solvent loss after annealing. Introducing an ambient rest period for completed devices and modules prior to encapsulation is therefore important in maximising device efficiency for these systems.Finally, the factors influencing device infiltration are explored in detail, with the aim of creating a reference resource of methods for targeted infiltration enhancement. A facile, non-destructive method for infiltration analysis is introduced and used to explore the impact of precursor crystallisation, ZrO2 roughness, ink rheology and carbon mesh marking on stack filling. These methods are then applied in well-performing stacks as a tool for general efficiency enhancement, resulting in a champion efficiency of 15% in a 1 cm2 device with the new solvent systems

Scalable Screen-Printed TiO2 Compact Layers for Fully Printable Carbon-Based Perovskite Solar Cells

Fully printable carbon-based perovskite solar cells (C-PSCs) represent some of the most promising perovskite solar cell (PSC) architectures. Highly scalable, stable, and low in cost—these devices consist of a TiO2 compact layer (C-TiO2) and three sequentially screen-printed mesoporous layers of TiO2, ZrO2, and carbon, through which perovskite is infiltrated. While there has been remarkable progress in optimizing and scaling up deposition of mesoporous layers and perovskite, few publications have focused on optimizing C-TiO2. In this work, we investigate the potential for substituting commonly used spray pyrolysis with more easily scaled screen-printing. It was found that when comparing layers of similar thickness, 1 cm2 devices fabricated with printed C-TiO2 exhibited similar power conversion efficiency (PCE) to those fabricated with spray pyrolysis. In contrast, thicker-printed C-TiO2 led to lower efficiency. The influence of TiCl4 treatment on the quality of produced compact layers was also examined. This proved beneficial, mostly in the printed films, where a champion PCE of 13.11% was attained using screen-printed, TiCl4 treated C-TiO2. This work proves that screen-printing is a viable replacement for spray pyrolysis in C-PSCs fabrication

Enhancing the Performance of the Mesoporous TiO2 Film in Printed Perovskite Photovoltaics through High-Speed Imaging and Ink Rheology Techniques

Mesoscopic carbon-based perovskite solar cells (C-PSCs) have the potential to be manufactured at an industrial scale by utilizing screen-printing, a simple, affordable, and commercially mature process. As such, many recent publications have focused on enhancing performance through modifying cell architecture and perovskite chemistries. This work examines how ink rheology can be tuned to optimize cell performance through reducing the occurrence of common print defects to create higher quality m-TiO2 films. Inks with different solvent dilutions and rheological profiles are assessed using high-speed imaging through the screen-printing visualization (SPV) technique, to investigate the impact of the viscosity and elasticity on ink separation mechanisms. The resultant film quality and its influence on device performances are then assessed. Ink separation lengths are minimized, and the formation of filaments ceases during printing, leading to improved TiO2 film topography and homogenous infiltration of the perovskite precursor. Consequently, PCE is improved by over 10% of the original efficiency in cells and 224 cm2 active area modules due to enhanced Voc and FF. These results not only provide key insights into tailoring ink rheology, to achieve improved print homogeneity and higher performing cells, but also aid further work on enhancing the performance of other screen-printed functional films

Investigating spatial macroscopic metastability of perovskite solar cells with voltage dependent photoluminescence imaging

Metastability is a characteristic feature of perovskite solar cell (PSC) devices that affects power rating measurements and general electrical behaviour. In this work the metastability of different types of PSC devices is investigated through current–voltage ( I – V ) testing and voltage dependent photoluminescence (PL-V) imaging. We show that advanced I – V parameter acquisition methods need to be applied for accurate PSC performance evaluation, and that misleading results can be obtained when using simple fast I – V curves, which can lead to incorrect estimation of cell efficiency. The method, as applied in this work, can also distinguish between metastability and degradation, which is a crucial step towards reporting stabilised efficiencies of PSC devices. PL-V is then used to investigate temporal and spatial PL response at different voltage steps. In addition to the impact on current response, metastability effects are clearly observed in the spatial PL response of different types of PSCs. The results imply that a high density of local defects and non-uniformities leads to increased lateral metastability visible in PL-V measurements, which is directly linked to electrical metastability. This work indicates that existing quantitative PL imaging methods and point-based PL measurements of PSC devices may need to be revisited, as assumptions such as the absence of lateral currents or uniform voltage bias across a cell area may not be valid

Quantifying Infiltration for Quality Control in Printed Mesoscopic Perovskite Solar Cells: A Microscopic Perspective

Mesoscopic carbon-based perovskite solar cells (CPSCs) are often cited as a potential frontrunner to perovskite commercialization. Infiltration, the extent to which perovskite fills the mesoporous scaffold, is critical for optimum performance and stability. However, infiltration data are usually presented as qualitative photographic comparisons of samples with extreme infiltration variation. This work examines how small infiltration defects impact performance using an optical microscopy examination of the base TiO2 layer to identify issues and develop targeted techniques for infiltration enhancement. Critically, the uninfiltrated area at the base of the stack was found to correlate well with PCE across multiple batches of varied print quality and ZrO2 thickness. Through reduction of mesh mark defects and improvement of print quality in the ZrO2 and carbon layers, a champion PCE of 15.01% is attained. It follows that this facile, multiscaled, nondestructive technique could enable targeted performance enhancement and quality control in future scale-up initiatives